Exhaust gas recirculation (EGR) systems recirculate a portion of exhaust gas from an engine exhaust to an engine intake system to improve fuel economy and vehicle emissions by reducing throttling losses and combustion temperatures. In turbo-charged direct injection engines, a low-pressure EGR (LP-EGR) circuit may be implemented. The LP-EGR circuit recirculates exhaust gases from an exhaust passage downstream of a turbine to an intake passage upstream of a turbocharger compressor.
However, due to the pre-compressor location of EGR delivery, there may a significant transport delay between the EGR valve and the combustion chamber. Specifically, the exhaust residuals may need to travel though the turbocharger compressor, high-pressure air induction plumbing, charge air cooler, and intake manifold before reaching the combustion chamber. As a result of the transport delay, during conditions when EGR needs to be rapidly reduced, such as during a tip-out to low load conditions, there may be more dilution in the intake than desired. The presence of increased intake-air dilution at low loads can increase combustion stability issues and the propensity for engine misfires.
One example approach for addressing the extra residuals is shown by Ma et al. in U.S. Pat. No. 6,014,959. Therein, a rigid connection is provided between an EGR throttle and a main air intake throttle, linking movement of the EGR throttle as a function of the movement of the main throttle. This allows EGR dilution to be always provided in a fixed proportion to the intake airflow.
However, the inventors herein have recognized potential issues with such an approach. As an example, the transport delay may not be sufficiently addressed while the fuel economy benefits of LP-EGR are limited. For example, the linking of EGR dilution to intake airflow may result in LP-EGR being provided at some low load points where no fuel economy benefit from the EGR is achieved. In some cases, there may even be a fuel penalty associated with the delivery of LP-EGR at the low load point. As such, it may not be possible to rapidly purge the LP-EGR from the intake in such systems without affecting airflow. As another example, the lower load points may limit the delivery of EGR at higher load points as they are the points where the combustion system is most dilution limited. As such, this can limit the peak EGR rates achievable during high loads. The presence of excess dilution in the engine intake system can also render the compressor susceptible to corrosion and condensation from the lingering EGR. Furthermore, increased condensation may occur at a charge air cooler of a boosted engine system due to the flow of EGR through the cooler. The increased condensation may necessitate additional counter-condensation measures.
The inventors have recognized that at least some of the above issues may be addressed by operating a hybrid vehicle system in a battery charging mode to rapidly purge LP-EGR. In one example, this is achieved by a method for a hybrid vehicle system comprising: in response to decreasing engine torque demand while operating an engine with EGR, disabling EGR, and until EGR in an engine intake is lower than a threshold, maintaining engine operation with EGR disabled and charging a system battery with the excess engine torque generated. In this way, LP-EGR can be rapidly purged without affecting torque to the wheels.
As an example, during medium to high load conditions, a hybrid vehicle system may be operated in an engine mode with the engine combusting to provide engine torque for propelling the vehicle wheels. Further, during the engine mode, low pressure EGR (LP-EGR) may be flowing from the engine exhaust to the engine intake to provide addition fuel economy and emissions benefits. In response to a tip-out to lower load conditions, EGR may be disabled by closing an EGR valve in an LP-EGR passage. By operating the engine with the EGR valve closed, EGR in the intake can be rapidly replaced with fresh intake air. As such, during the engine operation, the engine torque generated may be more than the demanded engine torque. The excess engine torque generated may be stored in a system battery if the battery has sufficient charge accepting capability. For example, the excess engine torque may be used to drive a motor/generator coupled to the battery. Engine operation and battery charging may be continued until the LP-EGR level is below a threshold level (e.g., all the LP-EGR has been replaced with fresh intake air). Thereafter, the engine may be shut down and the vehicle may be propelled via motor torque.
In this way, EGR purging from an engine intake can be expedited. By operating the engine with an EGR valve closed, intake EGR can be replaced with intake air. In addition, the higher engine torque can be advantageously used to charge a system battery. As such, this allows higher engine torque and higher battery charge to be held while EGR is purged. By rapidly reducing the intake EGR level at low load conditions, higher EGR rates can be achieved when the engine is subsequently restarted. As such, this substantially improves engine efficiency, particularly in medium to high engine speed-load regions. By replacing the EGR with fresh air, evaporation of water and hydrocarbon condensates is increased, reducing their concentration in the engine, and the need for counter-condensation measures. In addition, the reduction in condensation reduces compressor and charge air cooler corrosion and degradation. Overall, boosted engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.